It is well known in the metal-finishing art that metal surfaces such as ferrous and zinc surfaces may be provided with an inorganic phosphate coating by contacting said surfaces with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as an excellent base for the later application of siccative organic coating compositions such as paints, lacquers, varnish, primers, synthetic resins, enamels, rustproofing oil, etc.
The aqueous phosphating solutions utilized to deposit the desired phosphate coatings generally contain, in addition to phosphate ions, metallic ions such as zinc, lead, iron or manganese ions as the major cations. Other cations which may be present, generally in lower concentrations, include cobalt and nickel. Other auxiliary cations which may also be present in small amounts include sodium, cadmium, copper, and antimony ions. Anions which may be present in small amounts include halide ions such as bromide and fluoride ions, sulfate ions and borate ions. These auxiliary ions may be included in the phosphating solutions to influence the reaction with the metal surface, modify the character of the phosphate coating, modify the character of the sludge formed during the phosphating process and adapt the phosphating solution for a wide variety of applications. Other auxiliary agents which may be present include oxidizing agents, coloring agents, and metal cleaning agents.
The metal surfaces which may be treated with the phosphating solutions to provide protection against corrosion and a base for later application of top coats include nuts, bolts, screws, hood hinges and a variety of other parts. The desired coating is obtained generally by immersion of the parts in a hot solution of the phosphating solution. The first step in the phosphating of steel is the dissolving of the surface iron. The dissolved iron along with other ions present in the bath and formed during the phosphating process are converted to a sludge which is insoluble in the phosphating bath and settles to the bottom of the solution. The sludge must be periodically removed from the phosphating solution and transported, generally at significant cost for disposal at a landfill. It has become increasingly difficult and costly to dispose of the sludge.